Hierarchical micro/nanostructured silver hollow fiber boosts electroreduction of carbon dioxide

Efficient conversion of CO2 to commodity chemicals by sustainable way is of great significance for achieving carbon neutrality. Although considerable progress has been made in CO2 utilization, highly efficient CO2 conversion with high space velocity under mild conditions remains a challenge. Here, we report a hierarchical micro/nanostructured silver hollow fiber electrode that reduces CO2 to CO with a faradaic efficiency of 93% and a current density of 1.26 A · cm−2 at a potential of −0.83 V vs. RHE. Exceeding 50% conversions of as high as 31,000 mL · gcat−1 · h−1 CO2 are achieved at ambient temperature and pressure. Electrochemical results and time-resolved operando Raman spectra demonstrate that enhanced three-phase interface reactions and oriented mass transfers synergistically boost CO production.


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In addition, the Ag HF electrode was also treated with other different electrochemical oxidation times (30 s, 60 s, 120 s, 180 s, 300 s) at the fixed potential of 2.0 V (vs. Ag/AgCl), followed by the same electrochemical reduction at the fixed potential of -0.5 V (vs. Ag/AgCl) with the fixed reduction time (600 s) to obtain a series of activated Ag HF electrodes. After an overall comparison of the CO2 electroreduction performances, the activated Ag HF electrode obtained from Ag HF-redox-240s had the best electrocatalytic activity . Therefore, the activated Ag HF electrode in the main text and Supplementary Information refers to the electrode that underwent 240 s of oxidation and 600 s of reduction unless otherwise stated.

Synthesis of activated Ag foil
Ag foil and activated Ag foil working electrodes were used as references. A piece of Ag foil was ultrasonically cleaned in acetone and ethanol, and after drying in air, the side and back of the Ag foil were sealed with epoxy to obtain a Ag foil electrode with an exposure geometric area of 2 cm × 2 cm. And the synthesis procedure for activated Ag foil was the same as that of activated Ag HF. Thus, the activated Ag foil electrode also possessed the same exposed geometric area of 4 cm 2 .

Gas Permeation Tests
Gas permeation tests were performed with a custom gas permeability device that could record the permeability of H2, He, CH4, N2 and CO2 through the hollow fiber under different transmembrane pressure drops. According to the Yasuda-Tsai equations 1,2 , the permeability coefficient K of porous hollow fiber can be expressed as follows: = + − Eq.
(3) where K0 is the Knudsen permeability coefficient, B0 is the geometric factor of hollow fiber wall, P is the mean pressure on both sides of the fiber, and η is the viscosity of N2 gas. The values of K0 and B0 can be calculated from the slope and intercept of the plot of K to P. The effective porosity ( /q 2 ) can also be estimated by the Knudsen permeability coefficient K0 in Equation (4): 2 = ( . ) ( ) Eq. (4) where is the porosity, q is the tortuosity factor, R is the gas constant, T is the temperature, and M is the molecular weight of the gas.

CO2 Electroreduction and Product Quantifications
The potentiostatic electroreductions of CO2 over all electrodes were performed at ambient temperature and pressure on the Biologic VMP3 potentiostat using the gas-tight electrolysis cell, which comprised two symmetrical compartments made of quartz glass with an inner height of 5.0 cm, an inner length of 5.0 cm and an inner width of 1.5 cm (Supplementary Figs. 11b-d).
The cathodic and anodic compartments were separated by a Nafion 117 membrane, and the electrolysis cell was equipped with a KCl-saturated Ag/AgCl reference electrode in the cathodic compartment and a platinum mesh counter electrode in the anodic compartment.
CO2-saturated KHCO3 aqueous solutions with different concentrations were used as the electrolyte solutions, which were cycled in both the cathodic and anodic compartments at a fixed flow rate of 20 mL·min -1 by using two identical peristaltic pumps (Jihpump BT-50EA 153YX). Prior to the experiments, the electrolysis cell was vacuumized and then purged with CO2 for 30 min.

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Under the similar electrolysis conditions, CO2 flow rate of lower than 10 mL•min -1 resulted in very low CO faradaic efficiencies and CO2 conversion rates. While both the CO faradaic efficiency and CO2 conversion rate increased rapidly when CO2 flow rate was larger than 10 mL•min -1 , and up to 60 mL•min -1 . Further increasing the CO2 flow rate to more than 60 mL•min -1 led to the slow increase of CO faradaic efficiencies and the rapid decrease of CO2 conversion rate. In order to obtain both appropriate CO faradaic efficiency and CO2 conversion rate, the CO2 flow rate was fixed at 60 mL•min -1 during CO2 electroreduction unless otherwise stated ( Supplementary Fig. 13). In the situations with very large currents (>400 mA), the Biologic VMP3 potentiostat was connected to a VMP3 booster chassis with an option of 10 A current.
The retention time values of CO2 through the different electrodes have been estimated obeying the equations below based on their structure and porosity. That is the CO2 retention times through Ag HF and activated Ag HF are 31.6 and 30.5 ms, respectively. The retention time of the electrodes was calculated as follows: Eq. (6) = π ( ) Eq. (7) where τ is the retention time, Vwall is the pore volume of hollow fiber wall, Vin is the volume of hollow fiber inner channel, νCO2 is the flow rate of CO2, n is the number of hollow fiber tubes, is the porosity, q is the tortuosity factor, Dout is the outer diameter of hollow fiber, Din is the inner diameter of hollow fiber, and L is the length of hollow fiber.
The theoretical limits of CO partial current density, i.e., jCO,lim(gas) and jCO,lim(sol) were calculated by the following two Equations (8) and (9), respectively. The former jCO,lim(gas) is the theoretical limit of CO partial current density with all gas-phase CO2 molecules input into the electrolysis cell were electroreduced to CO. The latter jCO,lim(sol) is the theoretical limit of CO partial current density with all CO2 molecules dissolved in the electrolyte solution were electroreduced to CO 3-5 . , ( ) = Eq. (8) , ( ) = Eq. (9) where α is the number of transferred electrons for producing CO, F is the Faraday constant (96485 C•mol -1 ), S is the electrode area (4 cm 2 ), km is the mass transfer coefficient (km =1 to obtain the value of jCO,lim(gas)), νCO2 is the flow rate of CO2, Vm is the gas mole volume (24.5 L•mol -1 at 25 °C , 101.325 kPa), D is the diffusion coefficient of CO2 (2.02 × 10 -9 m 2 •s -1 ), c is the saturated bulk concentration of CO2 ( 34 mol•m -3 at 25 °C , 101.325 kPa), δ is the diffusion layer thickness, which is estimated to be 14.0 μm using the rotating disk electrode model with the Levich equation 3 .
The experimental CO2 conversion rate was determined in accordance with the following equation: = × % Eq. (10) The theoretical limit of CO2 conversion rate was calculated using Equation (11) below: For the long-term performance test of CO2 electroreduction, the fixed potential of -0.83 V (vs. RHE) was applied to the activated Ag HF electrode. The electrolyte was CO2-saturated 1.5 M KHCO3 and the CO2 flow rate was kept at 60 mL•min -1 . The catholyte and anolyte were cycled at a flow rate of 20 mL•min -1 , accompanied by the supplement of ultrapure water to maintain a constant concentration of 1.5 M KHCO3. The exhaust from the cathodic compartment was measured by the online gas chromatography (GC) during the whole 170-hour test.
All the current densities in the main text and Supplementary Information were based on the electrode geometric area.
Gas-phase products from the cathodic compartment were directly vented into a gas chromatograph (GC-2014, Shimadzu) equipped with a Shincarbon ST80/100 column and a Porapak-Q80/100 column using a flame ionization detector (FID) and a thermal conductivity detector (TCD) during the electroreduction tests and online analysis. A GC run was initiated every 15 min. To ensure the accuracy of the gas-phase products, when the CO concentration in the exhaust was lower than 10%, the FID detector was used for CO quantification; when the CO concentration in the exhaust was higher than 10%, the TCD was used as the main detector of CO, and the FID was used as the auxiliary detector. The TCD quantification was used for H2 quantification. All faradaic efficiencies reported were based on at least five different runs. High purity argon (99.999%) was used as the carrier gas of GC. In all the potentiostat electrolysis tests, H2 and CO were the only gas-phase products, and their faradaic efficiencies were calculated as follows: where Cproduct is the concentration of the gas-phase products (ppm), νCO2 is the flow rate of CO2 (60 mL·min -1 ), t is the reaction time, α is the number of transferred electrons for producing CO or H2, F is the Faraday constant, Vm is the gas mole volume, and Q is the total quantity of electric charge. The possible liquid-phase products from the cathodic compartment after potentiostatic electrolysis for 1 h were analyzed using an offline GC-2014 (Shimadzu) equipped with a headspace injector and an OVI-G43 capillary column (Supelco, USA). There were no liquidphase products detected by offline GC. The postreaction catholyte solution was also further analyzed by using a 600 MHz nuclear magnetic resonance (NMR) spectrometer (Bruker). After an hour of electrolysis, an aliquot of catholyte solution (0.5 mL) was mixed with 0.1 mL of DSS (6 mM) and 0.1 mL of D2O, which were used as internal standards. No liquid-phase product was detected by NMR. The diagram for the detailed fabrication procedures of Ag HF is shown in Supplementary  Fig. 1, and the related experimental descriptions can be found in the Materials and Methods section. The whole fabrication of Ag HF involved only basic laboratory apparatuses under relatively mild conditions. Notably, the above fabrication process produced one batch of Ag HF with a total length of more than 180 meters, demonstrating its high potential for scalable applications.
Supplementary Figure 2 | Electrochemical redox activation treatments of the Ag HF electrode to obtain different activated Ag HF electrodes, and their CO2 electroreduction performance. a, Electrochemical oxidation and reduction current density curves during different oxidation treatments (from 30 s to 300 s) at 2.0 V (vs. Ag/AgCl) in 0.5 M KHCO3, and the subsequent respective reduction treatments for 600 s at -0.5 V (vs. Ag/AgCl) in the same electrolyte solution. b, Comparison of the CO partial current densities of the Ag HF electrode and the different activated Ag HF electrodes. The CO and H2 faradaic efficiencies and total current densities over c, the Ag HF electrode and d-i, the different activated Ag HF electrodes in CO2-saturated 1.5 M KHCO3. Error bars in b-i were obtained from the average of six individual tests.
The Ag HF electrode was subjected to different oxidation treatments (30 s, 60 s, 120 s, 180 s, 240 s and 300 s) and subsequent respective reduction treatments for 600 s to obtain a series of activated Ag HF electrodes (see the aforementioned Preparations section for details), denoted as Ag HF-redox-30s, Ag HF-redox-60s, Ag HF-redox-120s, Ag HF-redox-180s, Ag HF-redox-240s,

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and Ag HF-redox-300s, respectively. From the oxidation and reduction current density-time curves of these activated Ag HF electrodes ( Supplementary Fig. 2a), the amounts of accumulated charge at different oxidation times were proportional to those in the corresponding reduction stages, implying the redox reactions highly obeyed aforementioned Equations (1) and (2), respectively.
The comparison of CO2 electroreduction performance over the Ag HF electrode and all the activated Ag HF electrodes is shown in Supplementary Fig. 2b, and their detailed CO and H2 faradaic efficiencies as well as the total current densities are also presented in Supplementary  Figs. 2c-i. One can see that the CO partial current density showed obvious superiority at the more negative potentials with increasing oxidation time. And the Ag HF-redox-240s electrode delivered the highest jCO among all the activated Ag HF electrodes. Therefore, the activated Ag HF electrode in the main text and Supplementary Information referred to the Ag HF-redox-240s electrode unless otherwise stated. The electrochemical oxidation and reduction treatments of Ag HF electrode to obtain the activated Ag HF electrode (Ag HF-redox-240s) were monitored by the time-resolved operando Raman spectroscopy. As shown in Supplementary Fig. 3, the Raman spectrum of Ag HF (at 0 s of the oxidation stage) showed the peaks at approximately 1012, 1360, 1603 and 1660 cm -1 , which were assigned to bicarbonate ions (HCO3 -) adsorbed at the electrode surface as the νHO-COO-, νsHOCOO-, νasHOCOO-and δHO-H (in H2O) modes, respectively, according to previous reports 6,7 . Once the oxidation reaction occurred (as short as 1 s of the oxidation stage), new Raman peaks appeared at 682, 1047, 1296 and 1517 cm -1 , which could be assigned to the as-formed Ag2CO3 species as βO-C-O, νCO3 2-, νsO-C-O and νasO-C-O, respectively, 8,9 besides bicarbonate ion related peaks. With increasing oxidation time (2 s to 7 s), the intensities of the Ag2CO3-ralted peaks increased rapidly, and reached the maximum at 8 s during the oxidation stage. Further increasing the oxidation time (8 s to 240 s), the intensities of the Ag2CO3-related peaks remained constant. Combining the electrochemical oxidation current density curve ( Supplementary Fig. 2) and the Raman observations, it was found that the oxidation reaction of Ag to Ag2CO3 occurred very quickly on Ag HF surface at the initial stage, and then expanded to the subsurface or substrate to some degree, which was responsible for the constant peak intensities while keeping the oxidation current densities of 40-120 mA· cm -2 after 8 s.
As for the subsequent electrochemical reduction process, the intensities of the characteristic Ag2CO3 peaks faded rapidly, and became very weak at 21 s of the reduction stage. The Ag2CO3related peaks were almost negligible at 24 s of the reduction stage and disappeared in the following reduction stage. Interestingly, the Raman observations on the electrochemical reduction were in consistence with the variation of the reduction current density curve ( Supplementary Fig. 2). That is the reduction current density of Ag HF-redox-240s decreased to zero after 24 s at the potential of -0.50 V (vs. Ag/AgCl) ( Supplementary Fig. 2). These timeresolved operando Raman results confirmed the transitions between Ag and Ag2CO3 compositions obeying Equations (1) and (2) during the electrochemical redox activation treatments of Ag HF to obtain activated Ag HF. Figure 4 | SEM images of the outer surface of a, Ag HF and b-g, different activated Ag HF electrodes with (a1-g1) low, (a2-g2) medium and (a3-g3) high magnifications, respectively. a, Ag HF, b, Ag HF-redox-30s, c, Ag HF-redox-60s, d, Ag HFredox-120s, e, Ag HF-redox-180s, f, Ag HF-redox-240s, and g, Ag HF-redox-300s.

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The outer surface morphologies of Ag HF and activated Ag HF with different redox pretreatments were investigated by SEM observations. As shown in the Supplementary Fig. 4a, the outer surface of Ag HF exhibited the abundant micrometer-sized pores with relatively smooth substrate. Once the electrochemical redox activation treatment even a slight oxidation as short as 30 s was applied, the outer surface morphology of the Ag HF-redox-30s changed greatly ( Supplementary Fig. 4b). Numerous nanorods covered the outer surface of Ag HF-redox-30s, making the pores indistinct. With increasing oxidation time, the outer surfaces of activated Ag HF electrodes exhibited increasing surface coarseness and decreasing diameter of the as-formed nanorods (Supplementary Figs. 4c-g). Note that these nanorods partly ordered and gathered at the outer surface of Ag HF-redox-240s ( Supplementary Fig. 4f). These hierarchical micro/nanostructures comprising partly ordered nanorods on the surface and micrometer-sized pores beneath the surface may maximized the three-phase reaction interfaces, resulting in the best electrocatalytic activity ( Supplementary Fig. 2). The morphologies of pristine Ag powder and as-prepared Ag HF were investigated by SEM observations, as shown in Supplementary Fig. 5. The particles in the Ag powder were spherical with a relatively even particle size (~ 60 nm), but they appeared to aggregate ( Supplementary Fig.  5a). In contrast, both the inner and outer surfaces of Ag HF showed a well-integrated substrate without spherical or granular particles ( Supplementary Fig. 5b), implying that the silver particles were completely sintered and fused to form an integral hollow-fiber base during the fabrication process, thereby benefiting mechanical strength reinforcement and electron transfer. Both the outer and inner surfaces of Ag HF possessed abundant irregular micrometer-sized pores with a pore size of 5-20 µm. TEM was used to further investigate the morphologies of Ag powder and Ag HF, as shown in Supplementary Fig. 6. From the low-magnification TEM image, the particles in the Ag powder were spherical with a particle size range of 20-120 nm ( Supplementary Fig. 6a), in agreement with the SEM observation ( Supplementary Fig. 5a), while the fused nanorod-like particles obtained by scraping off the outer surface of Ag HF were presented ( Supplementary Fig.  6b). Furthermore, the high-magnification TEM image showed that the lattice spacing of the Ag powder were 2.36 and 2.04 Å, corresponding to the (111) and (200) planes of metallic Ag, respectively ( Supplementary Fig. 6a). Ag HF also presented a lattice spacing of 2.36 Å, corresponding to the Ag (111) plane ( Supplementary Fig. 6b). These results indicate that Ag HF had the same metallic Ag phase as the pristine Ag powder. The cross-section morphologies and pore structures of Ag HF and activated Ag HF were studied by SEM, as shown in Supplementary Fig. 7. Ag HF and activated Ag HF possessed similar wall thicknesses of ~50 µm and outer diameters of ~425 µm; additionally, their pores in the wall were interconnected (Supplementary Figs. 7a1, b1). Different from the symmetrical outer and inner regions of Ag HF ( Supplementary Fig. 7a2), partly ordered nanorods gathered at the outer region of activated Ag HF, presenting a distinct configuration of hierarchical micro/nanostructures ( Supplementary Fig. 7b2) derived from the electrochemical redox activation treatments. That is the CO2 flow rate was kept at 10 mL•min -1 during the activation treatments and the redox reactions (referring to the aforementioned Equations (1) and (2)) occurred only at the outer region of the hollow fiber wall. Gas permeation was used to study the structural features of Ag HF and activated Ag HF before and after the reaction, as shown in Supplementary Fig. 8. All the gas permeances of H2, He, CH4, N2 and CO2 remained almost constant at different pressure drops, and the large permeance values indicated the high permeabilities of Ag HF and activated Ag HF before and after reaction. Moreover, the gas permeances were inversely proportional to the square roots of their molecular weight (the insets in Supplementary Fig. 8), implying that the gas transport mechanisms through all the hollow fibers were dominated by Knudsen diffusion 1,2 . Furthermore, the effective porosities of Ag HF and activated Ag HF, calculated by using the nitrogen permeance data according to Equation (4), were 38% and 32%, respectively. On the basis of the porosity and structure, the CO2 retention times through Ag HF and activated Ag HF were 31.6 and 30.5 ms, respectively. In addition, the postreaction activated Ag HF possessed an effective porosity of 29% and a CO2 retention time of 30.0 ms, close to those of activated Ag HF before the reaction, implying the structural stability due to the tough framework of activated Ag HF. The phase compositions of all involved silver samples were studied by XRD. As shown in Supplementary Fig. 9, Ag foil, Ag powder and Ag HF showed three diffraction peaks at 38.1°, 44.3° and 64.4°, corresponding to the (111), (200), and (220) planes of metallic Ag (JCPDS no.04-0783), respectively. After electrochemical oxidation treatment, in addition to metallic Ag peaks, many new peaks appeared in electrooxidized Ag HF, which were assigned to the various planes of Ag2CO3 (JCPDS no. 26-0339). This result indicated that the electrochemical oxidation reaction obeyed Equation (1). By the subsequent electrochemical reduction treatment, all Ag2CO3 peaks converted back to metallic Ag peaks in activated Ag HF. In addition, the postreaction activated Ag HF also presented the same phase compositions as the activated Ag HF before the reaction. These XRD results indicated that all involved silver samples had only a metallic Ag phase with the same crystal form except for the electrooxidized Ag HF intermediate. The surface compositions of all involved silver samples were studied by XPS. As shown in Supplementary Fig. 10, the Ag 3d spectra in Ag foil, Ag powder and Ag HF showed the main Ag 3d5/2 and Ag 3d3/2 core peaks at binding energies of 368.3 and 374.3 eV, respectively, indicating metallic Ag 0 characteristics. Regarding electrooxidized Ag HF, the Ag 3d5/2 and Ag 3d3/2 peaks were at 367.8 and 373.8 eV, respectively, corresponding to the characteristic peaks of Ag2CO3 (referring to the standard spectrum of silver carbonate). This result implied that the surface of electrooxidized Ag HF was covered with Ag2CO3. Furthermore, the XPS spectra of activated Ag HF before and after the reaction suggested the same metallic Ag 0 surfaces, indicating the stable metallic Ag 0 active component during CO2 electroreduction. Supplementary Figure 11 shows optical images of the Ag HF electrode and electrolysis cell in different views and states. The electrolysis cell comprised two symmetrical compartments made of quartz glass with an inner height of 5.0 cm, an inner length of 5.0 cm and an inner width of 1.5 cm. The Ag HF working electrode consisted of ten Ag HF tubes (i.e., Ag HF array), and each tube had an exposed length of 3 cm (Supplementary Fig. 11a). The working electrode and the Ag/AgCl reference electrode were in the cathodic compartment, and the Pt mesh counter electrode was in the anodic compartment ( Supplementary Fig. 11b). The cathodic and anodic compartments were separated by a Nafion 117 membrane ( Supplementary Fig. 11c). During CO2 electroreduction, CO2 penetrated through the wall of the activated Ag HF tubes via the copper tube, forming a large amount of bubbles ( Supplementary Fig. 11d). The detailed faradaic efficiencies of CO and H2 as well as the total current densities of activated Ag HF in different KHCO3 solutions are presented in Supplementary Figs. 12a-e. As the applied potential negatively shifted, the CO faradaic efficiencies decreased, while the H2 faradaic efficiencies and the total current densities rapidly increased, especially at more negative potentials. Moreover, CO faradaic efficiencies in low concentration KHCO3 solutions were higher than those in high concentration solutions at similar potentials. Furthermore, the CO partial current density showed superior in the relatively concentrated solutions with the best performance in 1.5 M KHCO3 (Supplementary Fig. 12f). Therefore, CO2-saturated 1.5 M KHCO3 aqueous solution was chosen as the electrolyte solution for CO2 electroreduction unless otherwise stated. By varying the CO2 flow rates, which further affect the retention time and mass transfer that are related to the main structural factors, the variations of electrocatalytic performance over the activated Ag HF electrode including CO2 reduction and HER processes have been clearly presented. As shown in Supplementary Fig. 13a below, the CO2 flow rate significantly influenced the product faradaic efficiency and CO2 conversion rate over activated Ag HF electrode under the constant current density of 1.2 A•cm -2 . Only H2 was detected when no CO2 flowed through the porous electrode, indicating the dominant HER. With increasing CO2 flow rates, the H2 faradaic efficiencies monotonically decreased, and the CO faradaic efficiencies correspondingly increased resulting in a total faradaic efficiency of 100%. This implies that high local CO2 concentration generated by the sufficient CO2 flow suppressing HER while facilitating CO2 reduction. However, compared to the CO faradaic efficiency, the CO2 conversion rate exhibited different variations with respect to CO2 flow rate. That is the CO2 conversion rate increased rapidly at first with the gradually increasing CO2 flow rates, and a maximum conversion of 68% was yielded at 40 mL•min -1 with a CO faradaic efficiency of 81%. Interestingly, the CO2 conversion rate faded with further increasing CO2 flow rates, even down to 32% at 100 mL•min -1 . These results imply that CO2 reduction kinetics may also be affected by the electrode intrinsic structures besides the competitive HER.

Supplementary
Furthermore, variations on the faradaic efficiency ratio of FECO/FEH2 and the CO2 conversion rate with respect to the retention time (obtained from Equations (5), (6) and (7) on basis of the electrode intrinsic structure characteristics) under the constant current density of 1.2 A•cm -2 can be clearly seen in Supplementary Fig. 13b. The FECO/FEH2 ratio increased when the retention time decreased. Regarding the theoretical limit of CO2 conversion rate, i.e., ConCO2,lim (referring to Equation (11)), it remained at 100% in the retention time range from 183 to 46 ms (corresponding to CO2 flow rates from 10 to 40 mL•min -1 ), and then decreased rapidly with the further decreasing retention time. In fact, the experimental results of CO2 conversion rates were quite low at long retention time situations and then close to the theoretical values at short retention time situations. In order to obtain both appropriate CO faradaic efficiency and CO2 conversion rate, the CO2 flow rate was fixed at 60 mL•min -1 during CO2 electroreduction unless otherwise stated.
In addition, the theoretical limit and experimental values of CO partial current density under different CO2 flow rates were further studied. Actually, there are two kinds of theoretical limits of CO partial current density: (1) all gas-phase CO2 molecules input into the electrolysis cell are reduced to CO with a 100% conversion rate, i.e., jCO,lim(gas), which is calculated using above Equation (8); (2) all CO2 molecules dissolved in the electrolyte solution are reduced to CO, i.e., jCO,lim(sol), which is calculated using above Equation (9). As shown in Supplementary Fig. 13c, the experimental jCO values over activated Ag HF were far larger than those of jCO,lim(sol). Although the experiment results were still lower than those of jCO,lim(gas) as an extremely ideal case, the activated Ag HF delivered a maximum jCO of 1.40 A•cm -2 at 60 mL•min -1 , superior to the previous reports (Supplementary Table 1). In contrast, all jCO values over activated Ag foil (Fig. 4b) were lower than those of the theoretical jCO,lim(sol). Moreover, on basis of Equation (8), the mass transfer coefficients were 0.4, 0.6, 0.7 and 0.7 over activated Ag HF, corresponding to the CO2 flow rates of 10, 20, 40 and 60 mL•min -1 , respectively, which were much larger than those over activated Ag foil. Typical GC curves from the FID and TCD over activated Ag HF are shown in Supplementary Fig. 14. When the CO concentration in the exhaust was lower than 10%, the FID was used for CO quantification (Supplementary Fig. 14a). When the CO concentration in the exhaust was higher than 10%, the TCD was used as the main detector of CO, and the FID was used as the auxiliary detector ( Supplementary Fig. 14b). The TCD was always used for H2 quantification. Moreover, H2 and CO were confirmed to be the only gas-phase products. The 1 H-NMR spectrum of the postreaction catholyte solution after 1 h of CO2 electroreduction over activated Ag HF at -0.83 V further verified that no liquid-phase product could be detected ( Supplementary Fig. 14c). As shown in Supplementary Fig. 15, the CO2 conversion rates of activated Ag HF were comparable to those over prominent catalysts reported in electrocatalysis in the potential range of -0.35 to -0.70 V. With negative-shifting potentials, the CO2 conversion rates over the activated Ag HF electrode further increased rapidly and reached 28%, 37%, 54% and 65% at -0.72 V, -0.75 V, -0.83 V and -0.89 V, respectively ( Supplementary Fig. 15), far outperforming the previously reported electrocatalysts (Supplementary Table 1).

Supplementary Figure 16 | ECSA measurement results.
Cyclic voltammetry curves of a, Ag foil, b, activated Ag foil, c, Ag HF, and d, activated Ag HF in CO2-saturated 1.5 M KHCO3. e, Plot of Δj (the difference of cathodic and anodic current densities, jc-ja) against the scan rates from cyclic voltammetry curves. The plots in Supplementary Fig. 16e, same as Fig. 4a in the main text. All the current densities in the main text and Supplementary Information were based on the electrode geometric area.
The ECSAs of Ag foil, activated Ag foil, Ag HF and activated Ag HF were determined by measuring their double-layer capacitance (Cdl) values via their cyclic voltammetry curves, as S26 shown in Supplementary Figs. 16a-d. The Cdl, which was proportional to the ECSA, was obtained by linearly fitting the absolute value of the slope of Δj (the difference of cathodic and anodic current densities of the cyclic voltammetry curves) against the scan rates. Activated Ag HF possessed the largest ECSA with a Cdl value of 30.9 mF•cm -2 , and this value was 2.7, 2.7 and 10.3 times those of activated Ag foil (11.4 mF•cm -2 ), Ag HF (11.3 mF•cm -2 ) and Ag foil (3.0 mF•cm -2 ), respectively ( Supplementary Fig. 16e).

Supplementary Figure 17 | Electrocatalytic performance of activated Ag HF and other
counterparts. CO and H2 faradaic efficiencies, and total current densities over the electrodes a, Ag foil, b, activated Ag foil, c, Ag HF, and d, activated Ag HF in the potential range of -0.35 to -1.15 V in CO2-saturated 1.5 KHCO3. e, Comparison of the CO partial current densities over these electrodes. The plots in Supplementary Fig. 17e, same as Fig. 4b in the main text. Error bars in a-e were obtained from the average of six individual tests.
Ag foil showed very low CO2 electroreduction activity, and the CO faradaic efficiencies at all potentials were less than 22% (Supplementary Fig. 17a). After electrochemical redox treatments, the CO faradaic efficiencies of activated Ag foil improved to some degree ( Supplementary Fig. 17b). At -0.50 V, the CO faradaic efficiency of activated Ag foil reached a maximum of 57%. While activated Ag foil also delivered CO faradaic efficiencies that were far less than 50% at other potentials ( Supplementary Fig. 17b). The results implied that the hydrogen evolution reaction was still dominant with the activated Ag foil. As shown in Supplementary Fig.  17c, Ag HF showed slightly better CO2 electroreduction activity than activated Ag foil, i.e., higher faradaic efficiencies and total current densities at similar potentials. With respect to activated Ag HF, all of the CO faradaic efficiencies and total current densities increased greatly ( Supplementary Fig. 17d). The comparison of the CO partial current densities indicated the obvious superiority over the activated Ag HF electrode compared with the other electrodes ( Supplementary Fig. 17e).

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Supplementary Figure 18 | CO2 electroreduction path. Reaction steps for the electroreduction of CO2 to CO on silver catalysts. Supplementary Figure 18 shows a possible mechanism for CO2 electroreduction on silver catalysts, in which the reaction paths proposed were consistent with previous reports 10, 11 . The initial step (Step 1) in the overall two-electron reduction of CO2 to CO on the silver surface was a one-electron transfer step forming adsorbed *COO -. Then, Step 2 was a chemical step involving the protonation of *COOto form a *COOH intermediate. Subsequently, Step 3 was an electrochemical path coupled to a chemical reaction involving the proton-electron transfer and instantaneous dehydration to form an adsorbed *CO intermediate. Finally, Step 4 was the desorption of *CO from the silver surface to obtain the CO product. The Tafel slope of activated Ag foil was 113 mV•dec -1 , close to that (108 mV•dec -1 ) of activated Ag HF with the non-CO2disperser mode (Fig. 5c), implying that Step 1 with the theoretical value of 118 mV•dec -1 12 was the rate-determining step for both electrodes. In contrast, activated Ag HF with the CO2disperser mode showed a Tafel slope as low as 63 mV•dec -1 , suggesting Step 1 was not the ratedetermining step. In principle, each one of Step 2, Step 3 and Step 4 was probably to be the ratedetermining step of activated Ag HF with the CO2-disperser mode. According to the previous report 12,13 , if Step 3 was the rate-determining step, the Tafel slope will be generally less than 40 mV•dec -1 . In addition, if Step 4 was the rate-determining step, the Tafel slope will be ∞ (infinity) 13,14 . Thus, the Tafel slope value (63 mV•dec -1 ) of activated Ag HF with the CO2disperser mode ruled out the situations of Step 3 and Step 4 as the possible rate-determining steps. Consequently, Step 2 with the theoretical value of 59 mV•dec -1 was the rate-determining step 13,14 for activated Ag HF with the CO2-disperser mode, in agreement with many reports [15][16][17] . The result meant that the CO2-disperser mode of activated Ag HF played a crucial role in CO2 electroreduction, which might induce the synergistic effects to alter the route of CO2 reduction. Isotopic trace experiments were conducted to study the mass migrations involved in CO2 electroreduction over activated Ag HF. For comparison, the feedstocks were supplied into the cathodic and anodic compartments of the electrolysis cell according to the below four situations, respectively, which were subjected to the potentiostatic electrolysis under the same reaction conditions (see the Materials and Methods section for details).
For both activated Ag HF and activated Ag foil, typical operando Raman spectra in a wide range of 300-2000 cm -1 showed two kinds of Raman peaks above and below 1000 cm -1 , respectively, as shown in Supplementary Fig. 20. The Raman peaks above 1000 cm -1 included four peaks centered approximately at 1012, 1360, 1603 and 1660 cm -1 , which were assigned to bicarbonate ions (HCO3 -) adsorbed on the electrode surface as νHO-COO-, νsHOCOO-, νasHOCOO-and δHO-H (in H2O) modes, respectively, according to previous reports 6,7 . The other Raman peaks below 1000 cm -1 showed only two Raman bands at 532 and 390-410 cm -1 , which could be assigned to the adsorbed intermediate vibrations, i.e., ν*COO-and νAg-*COOH, in consistence with previous reports 18,19 . The lower νAg-*COOH frequency (393 cm -1 ) of activated Ag HF compared with that (408 cm -1 ) of activated Ag foil suggested a weaker bonding strength between *COOH and the activated Ag HF surface. Furthermore, the *COOand *COOH intermediates appeared in chronological order in the time-resolved operando continuous Raman spectra ( Fig. 6b and Supplementary Fig. 21c), implying the step-by-step reduction of CO2, i.e., the initial step to form *COOand the second step to form *COOH, in agreement with the proposed mechanism ( Supplementary Fig. 18). Considering its higher sensitivity and intensity, the *COOintermediate was given more attention in the following.
For comparison of the relative intensity of *COOin these two electrodes, namely, activated Ag HF and activated Ag foil, we estimated the relative ratio of the integral peak areas of adsorbed *COOand aqueous HCO3 -(i.e., νasHOCOO -+ δHO-H), which are marked with shadows in Supplementary Fig. 20. The *COO -/(νasHOCOO-+ δHO-H) ratio was 6.7 for activated Ag HF after power-on for 2720 ms, and this ratio value did not change in the following stable state. In contrast, this ratio in the stable state was 3.3 for activated Ag foil, which was only half of that of activated Ag HF. This result implied that more *COOintermediates were formed and adsorbed on the surface of activated Ag HF. The formation and evolution of key intermediates over activated Ag HF and activated Ag foil were monitored by time-resolved operando Raman spectroscopy during the power-on and power-off stages, respectively ( Supplementary Fig. 21). After power-on for 720 ms (t1), a new Raman peak appeared at 532 cm -1 over activated Ag HF, corresponding to the adsorbed *COOintermediate ( Supplementary Fig. 20). Then, the peak intensity increased quickly and reached the maximum at 2720 ms (t2) (Supplementary Fig. 21a and Fig. 6d). Regarding activated Ag foil, the *COO -Raman peak appeared at 660 ms (t1'), and the peak intensity reached a maximum at 3080 ms (t2') ( Supplementary Fig. 21c and Fig. 6d). Notably, the normalized *COOpeak intensity of activated Ag HF was almost double that of activated Ag foil in the stable state. These results indicated that more *COOintermediates were formed and adsorbed over activated Ag HF in a shorter time, implying the superior capability of CO2 activation, which probably profited from the reduced CO2 diffusion distance in the CO2-disperser mode.
Subsequently, we investigated the variation of adsorbed *COOover activated Ag HF and activated Ag foil during the power-off stages (Supplementary Figs. 21b, d. As soon as the power was turned off, the 532 cm -1 ν*COO-peak quickly redshifted for both electrodes due to the Stark effect [20][21][22] , indicating the distinct impact of electric field on the adsorption of intermediates ( Supplementary Fig. 22). Then, the intensity of the *COO -Raman peak decreased gradually. The *COOpeak vanished over activated Ag HF after power-off for 1050 s (t3) (Supplementary Fig.  21b and Fig. 6d), whereas over activated Ag foil after power-off for 1400 s (t3') (Supplementary Fig. 21d and Fig. 6d), indicating a faster dissipation of adsorbed *COOover activated Ag HF. This result implied that the one-way CO2 flow manner of activated Ag HF facilitated the desorption of adsorbed intermediates or species on its surface (vide infra).
The above time-resolved operando Raman results suggested that the oriented mass transfers induced by the CO2-disperser mode of activated Ag HF could not only favor the diffusion of CO2 to active sites but also facilitate the desorption of adsorbed species from the electrode surface, thereby resulting in the improved overall kinetics of CO2 reduction. Consequently, activated Ag HF also demonstrated the promotion in mass transfers of CO2 electroreduction in addition to enhanced three-phase interface reactions. Figure 22 | Electric field impact on Raman spectra. Raman spectra (300−600 cm −1 ) of activated Ag HF and activated Ag foil during frequent switching between power-on and power-off.

Supplementary
To study the Stark effect of electric field on the adsorbed intermediates, Raman spectra over activated Ag HF and activated Ag foil were recorded during frequent switching between power-on and power-off, as shown in Supplementary Fig. 22. In power-on situations from power on-1st to power on-5th, all *COO -Raman peaks were located at 532 cm -1 for both activated Ag HF and activated Ag foil, while the *COOpeak quickly and consistently redshifted for both electrodes when switched to power-off, indicating the reproducible occurrences of the Stark effect [20][21][22] . This result indicated that electric field could play a crucial impact on the adsorption of surface intermediates. In detail, ν*COO-redshifted to 512 cm -1 over activated Ag HF, whereas to 519 cm -1 over activated Ag foil. Note that there was a 7 cm -1 shift in *COOvibration peak between activated Ag HF and activated Ag foil during all power-off situations. The lower frequency suggested the weaker interaction of *COOwith the surface of activated Ag HF, which was another sign of its easier desorption of adsorbed intermediates or species when compared to activated Ag foil. These results implied that activated Ag HF was intrinsically favorable for the desorption of adsorbed surface species. The desorption of *COOover activated Ag HF with non-CO2-disperser mode was also monitored by time-resolved Raman spectra. As shown in Supplementary Fig. 23, the *COOpeak vanished over activated Ag HF with non-CO2-disperser mode after power-off for 1380 s (t3''), which was close to the dissipation time of 1400 s (t3') over activated Ag foil ( Supplementary Fig. 21d). This result also demonstrate that the CO2-disperser mode played a key role for the desorption of adsorbed species.